Pathological anatomies such as tumors and lesions can be treated with an invasive procedure, such as surgery, which can be harmful and full of risks for the patient. A non-invasive method to treat a pathological anatomy or other target is external beam radiation therapy. A “target” as discussed herein may be an anatomical feature(s) of a patient such as a pathological anatomy (e.g., tumor, lesion, vascular malformation, nerve disorder, etc.) or normal anatomy and may include one or more non-anatomical reference structures. In one type of external beam radiation therapy, an external radiation source is used to direct a sequence of x-ray beams at a tumor site from multiple angles, with the patient positioned so the tumor is at the center of rotation (isocenter) of the beam. As the angle of the radiation source changes, every beam passes through the tumor site, but passes through a different area of healthy tissue on its way to the tumor. As a result, the cumulative radiation dose at the tumor is high and the average radiation dose to healthy tissue is low.
The term “radiotherapy” refers to a procedure in which radiation is applied to a target region for therapeutic, rather than necrotic, purposes. For convenience, the term “radiation treatment” is used herein to include radiosurgery and/or radiotherapy unless otherwise noted.
Conventional radiation treatment can be divided into at least two distinct phases: treatment planning and treatment delivery. A treatment planning system may be employed to develop a treatment plan to deliver a requisite dose to a target region, while minimizing exposure to healthy tissue and avoiding sensitive critical structures. A target region may be a tumor. Alternatively, the target region may be another pathological anatomy. Surrounding tissue may be soft tissue or critical structures. Some examples of critical structures include vital organs, bones, and other physical structures that may be affected by radiation treatment. A treatment delivery system may be employed to deliver the radiation treatment according to the treatment plan. Treatment plans specify quantities such as the directions and intensities of the applied radiation beams, and the durations of the beam exposure. A treatment plan may be generated from input parameters such as beam positions, beam orientations, beam shapes, beam intensities, and radiation dose distributions (which are typically deemed appropriate by the radiologist in order to achieve a particular clinical goal). Sophisticated treatment plans may be developed using advanced modeling techniques and optimization algorithms. Treatment planning procedures, such as forward and inverse planning are conventionally known.
Some conventional radiation systems attempt to optimize the treatment plan prior to delivery. One such radiation system is the TomoTherapy Hi-Art System® available from TomoTherapy, Inc., of Madison, Wis. The Hi-Art System facilitates optimization of the treatment plan by calculating a planned dose into a phantom and then measuring a dose delivered into the phantom. Although such a system may facilitate optimization of the treatment plan during the treatment planning stage, it does not optimize radiation treatment based on the radiation actually absorbed at the target region during the treatment delivery stage.
Whether forward planning or inverse planning is used, conventional treatment plans assume specific treatment conditions. However, the actual treatment conditions during treatment delivery are typically different from the treatment planning assumptions. Such differences are not reflected in the treatment plan because they are unknown at the time of treatment planning and may result in an error between the planned radiation dose and the actual radiation dose. Conventional radiation treatment systems allow such deviations as acceptable tolerance errors and do not determine or generate any kind of record of the error. Furthermore, conventional radiation treatment systems do not allow the treatment delivery to be modified based on the difference between the planned dose and the actual dose delivered.
In particular, since it is seldom possible to measure dose distribution directly in patients treated with radiation, conventional radiation treatment systems derive model data on dose distributions from measurements in phantoms—tissue equivalent materials. The model data is used to predict dose distribution in an actual patient during treatment. For example, in one conventional radiation treatment system, the system can estimate the dose based on the accumulated depth, which is based on the tissue the radiation beam passes through. The model can use the actual depth and/or the accumulated depth (based on the tissue equivalent materials in the path of the radiation beam) to estimate the dose. However, it should be noted that the actual depth and accumulated depth used in treatment planning are assumed to be fixed values. The dose absorption may be estimated using the fixed values that are derived from a static CT image, which corresponds to a single instance in the breathing cycle of the patient. During treatment delivery the motion of the target and/or surrounding tissue may change the actual depth of the target region with respect to the surface of the body. Since the actual depth has been assumed during treatment planning to be fixed, motion that changes the actual depth during treatment planning may impact the actual dose absorbed at the treatment target. Conventional radiation treatment system do not take into account the internal movements of the target and the surrounding tissue with respect to one another and the surface of the body for determining the dose absorbed at the target region, but merely estimate the dose absorbed based on a fixed target depth.